This invention relates to a zoom objective for use in a 8 mm cinematograph camera or television camera, and which has a high relative aperture, covers a generally accepted range of variation of equivalent focal length, and is particularly of low cost. More particularly, it relates to a zoom objective having a relative aperture ratio of f/1.8 and a zoom ratio of 3 and covering a field angle of as large as 52.degree. while nevertheless the focusing provision is made at only one lens element.
Of various types of zoom objectives such as those adapted for use in projectors, there is known a type of zoom objective of which the front focusing member consists of only one lens element. Usually such an objective is designed to have a zoom ratio which is in the order of 1.5 to 2 and a field angle of not more than 40.degree.. Accordingly, the features of the zoom objective abovespecified assist in facilitating the solution of aberrational problems. On the other hand, it has generally been recognized in the art that increasing of the zoom ratio and the field angle to 3.degree. and 50.degree., respectively, as in the zoom objective according to the invention, makes it more difficult to achieve a good state of correction of the chromatic image aberrations throughout the zooming range, unless each of the movable lens members in the zoom objective satisfies the achromatic condition.
Attempts have been made to reduce the production cost of zoom objectives by decreasing the number of lens elements in a single objective. A typical zoom objective of such low cost design comprises a first member arranged to serve as a focusing member and as a compensator, a second member which is a variator, and a relay system. In order for the movable lens members to satisfy the achromatic condition, a compound component is employed in the first member. In this arrangement, however, it is necessary to impart increased curvature to some of the surfaces of the lens elements of the objective and particularly to the cemented contact surfaces in the compound components, whereby it is made impossible to grind a great number of lens elements, having such large radii of curvature, at one time. Therefore, the decrease in the number of lens elements which can be produced in a single production run results in an increase in the production cost which cancels the decrease in the production cost of completed objectives effected by the decrease in the number of lens elements in the single zoom objective.